This invention is in the field of communications circuitry, and is more specifically directed to phase-locked loop circuitry in multiple frequency band applications.
Many advances have recently been made in the field of telecommunications, particularly in digital wireless telecommunication technology. In particular, modern digital telephones now operate at extremely high frequencies, thus providing high quality portable voice and data communications. These advantages in digital wireless telecommunication are expected to result in widespread use of this technology, as well as in new applications for digital wireless communication.
As is well known in the field of digital telecommunications, the operation of each transceiving device, such as a telephone handset, must be synchronized with the digital signals being communicated. This synchronization takes place at a selected channel within a frequency band specified by the particular communications protocol. For example, in one high-frequency digital communications protocol, three hundred seventy-four channels are specified for transmission, from a handset to the network, within a band of 1710 MHz to 1785 MHz; similarly, three hundred seventy-four receive channels are specified over a different band (e.g., between 1805 MHz and 1880 MHz). The use of multiple channels, of course, permits a large number of communications to be simultaneously ongoing, with a minimum of interference. Synchronization of the handset to the selected channels is, of course, necessary to ensure proper communication.
A well known circuit for synchronizing the operation of a system to a periodic signal is the phase-locked loop. Fundamentally, a phase-locked loop includes a phase detector for determining the temporal relationship of the phase-locked loop output signal, on one hand, to a reference signal with which the phase-locked loop is to be synchronized. The phase detector circuit controls a charge pump in a manner corresponding to the phase relationship of the two signals. The charge pump charges or discharges a capacitor in a low-pass loop filter in response to the phase detector output. The filtered voltage is applied to a voltage-controlled oscillator (VCO), which generates a periodic signal at a frequency determined by the charge pump output. Accordingly, the output of the phase detector determines whether the frequency of the VCO is to be advanced or retarded; continued operation eventually results in the VCO output "locking-in" upon the reference signal.
In some instances during a digital wireless communication, the phase-locked loop must perform its synchronizing function very rapidly. For example, if the portable digital telephone moves from one "cell" to another during a transmission, the phase-locked loop may have to rapidly change operating frequency from one channel to another within the band to accomplish "handing off" of the communication from cell-to-cell. As such, it is important for the phase-locked loop circuitry in a modern digital telephone to rapidly respond in changing operational frequencies from channel to channel within a band. Conventional telecommunications specifications and standards typically specify a time within which the phase-locked loop must synchronize to a new frequency.
One well-known telecommunications standard, referred to as the GSM digital telecommunications standard, requires digital telephone equipment to be operable in either of two frequency bands. In this example, one frequency band specifies transmissions to be made by the handset at frequencies between about 880 MHz and 915 MHz, and for signals to be received by the handset at frequencies between about 925 MHz, and 960 MHz; a second frequency band specifies handset transmissions at frequencies between about 1710 MHz and 1785 MHz, and communications received by the handset at frequencies between about 1805 MHz and 1880 MHz. Accordingly, each telephony device must be capable of operating within either of these frequency bands in order to satisfy the GSM digital communications standard.
Because of the wide separation between the two GSM frequency bands (the higher frequency band being at approximately twice the frequency of the lower frequency band), a single conventional VCO cannot accurately deliver frequencies over both of the bands. As such, a brute force way to implement the GSM standard in telephonic equipment would be to incorporate two PLLs, one for each band, duplicating the VCO, phase detector, charge pump, and loop filter. In other applications, even in cases where the frequency bands are relatively close, differences in lock-in times or in acceptable phase noise levels may necessitate the use of two PLLs. In any event, this brute force approach requires double the integrated circuit chip area for the implementation of both PLLs, which is of course inefficient.